The instant invention relates to a method for inhibiting the corrosion of metallic surfaces in contact with aqueous systems and to compositions for use in such a method, particularly where the water of the aqueous system is oxygen-bearing. More particularly, the present invention relates to the use of compositions comprising a combination of a molybdate ion surface and a component selected from the group consisting of water soluble polymers of polymaleic acid or anhydride and amine adducts thereof, maleic anhydride copolymers, water soluble polymers containing a sulphonic acid and carboxylic acid moiety, salts of the above-described polymers, phosphonates, phosphino carboxylic acids, polyphosphoric acid and glycol esters of polyphosphoric acid, to inhibit the corrosion of metallic surfaces of water-carrying systems.
The term "aqueous system" as used herein, is intended to describe any system which contains water in any physical state, including water which contains one or more dissolved or dispersed substances such as inoroganic salts.
The term "metallic" as used herein, is intended to include ferrous and ferrous-containing materials.
The corrosion of a metallic surface in an aqueous system consists of the destruction of the ferrous metal by chemical or electrochemical reaction of the metal with its immediate environment.
Where the corrosion is electrochemical in nature, a transfer or exchange of electrons is necessary for the corrosion reaction to proceed. When corrosion of the metal takes place, at least two electrochemical processes occur, and must occur, simultaneously. There is an anodic oxidation reaction in which metal ions go into solution, leaving behind electrons; and at least one cathodic reduction reaction in which species in solution are reduced by consuming the electrons produced by the anodic reaction. With respect to ferrous or ferrous containing materials, when the water contains oxygen and is at a neutral pH or above, these processes may be illustrated by the following equations:
Anodic oxidation:
Fe.fwdarw.Fe.sup.+2 +2e.sup.- PA1 (a) a molybdate ion source; and PA1 (b) a water-soluble component selected from the group consisting of polymaleic anhydride, amine adducts of polymaleic anhydride, polymers prepared by polymerizing maleic anhydride with dimethyl diallyl ammonium chloride or homologs thereof, polymers containing carboxylic acid and sulphonic acid moieties, salts of the above-described polymers, phosphonates, phosphino carboxylic acids, polyphosphoric acid and glycol esters of polyphosphoric acid. PA1 (a) 40 to 95%, by weight, of an unsaturated carboxylic compound selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, itaconic acid, their salts and mixtures thereof; PA1 (b) 5 to 60%, by weight, of an unsaturated sulfonic compound selected from the group consisting of 2-acrylamido-2-methylpropylsulfonic acid, 2-methacrylamido-2-methylpropylsulfonic acid, methallylsulfonic acid, allylsulfonic acid, vinyl sulfonic acid, styrene sulfonic acid, their salts and mixtures thereof; and PA1 (c) 0 to 40%, by weight, of an unsaturated polyalkylene oxide compound.
Cathodic reaction: EQU 2H.sub.2 O+O.sub.2 +4e.sup.- .fwdarw.40H.sup.-
The two ionic reaction products, ferrous ion and hydroxyl ion, combine to form ferrous hydroxide, Fe(OH).sub.2, whichis then oxidized to form ferric hydroxide, Fe(OH).sub.3 (rust). For ferrous or ferrous-containing materials as well as other metals in aqueous systems, the principle factors influencing the corrosion process are the characteristics of the water in the system, including but not limited to the rate of water flow, the temperature of the system and contact between dissimilar metals in the system. Variable characteristics of the water which impact upon its corrosiveness are its dissolved oxygen concentration, carbon dioxide contant, pH and hardness.
The presence of dissolved oxygen in the water of an aqueous system is primarily the result of contact between the water and the atmosphere. The oxygen solubility in water is temperature and pressure dependent, with increases in pressure increasing solubility and increases in temperature lowering oxygen solubility.
Corrosion produced by the presence of oxygen in the water of an aqueous can take place in the form of small pits or depressions and/or in the form of general metal loss. As a corrosive process continues, pits or depressions generally increase in depth. The corrosive attack is more severe when it causes pits or depressions, since the deeper penetration of the metal causes more rapid failure at these points.